Floating vessels (ships, platforms, etc.) are commonly used for drilling, servicing and maintenance of subsea oil and gas wells. Typically, a riser is suspended underneath a drill floor and extends to a subsea wellhead on the seabed. A drill string may be suspended by the drilling derrick and run inside the riser, through the wellhead and into a subterranean hydrocarbon reservoir. The distance (and hence drill string length) between the seabed wellhead and the reservoir may be considerable. In this configuration, the riser is fixed to the seabed (via the wellhead), while the drill string is not. A malfunctioning drill string or drill string compensator will therefore normally not compromise the integrity of the well, as the drill string runs inside the riser. The riser ensures that well is not open to the seawater.
The respective connections between the riser and vessel and between the drill string and the vessel must be compensated for the vessel's movement in the water. The predominant factors for causing vessel movements are waves and tidal currents, but drift could also be a factor if the vessel is not firmly anchored to the seabed. The distance between a fixed point on the vessel and a seabed wellhead will vary according to the magnitude of these factors.
Compensators are generally based on pressurized cylinders in a hydraulic-pneumatic system. This so-called passive compensator is in effect a spring with a predetermined (albeit adjustable) force. A passive compensator will in principle require no external utilities (e.g. electricity, control system, air or oil supply) during operation. The riser is normally suspended by a tensioner system underneath the drill floor. The drill string is normally suspended by a drill string compensator (hence often referred to as a “DSC”) at the top of the derrick (“top-mounted compensator”), which is commonly known in the art
In another operational configuration, the drill string (or casing) extends between the vessel and the seabed without a riser. The drill string may be connected to a x-mas tree and may in a context of compensation be considered to be fixed to the seabed. In this so-called “fixed-to-bottom” configuration, the compensator capacity requirement is reduced considerable, as the drill string only extends to the seabed and not into the well. However, having the riserless drill string in a fixed-to-bottom configuration is a precarious condition, in that the well will become open to the surrounding seawater if the drill string should fail, for example due to compensator malfunction. The reliability of the compensator system is therefore highly critical factor in this configuration.
The state of the art in drill string compensators includes a passive top-mounted drill string compensator (DSC) arranged at the top of the derrick. This drill string compensator is connected to the crown block (hence also often referred to as a “crown-mounted compensator”, or “CMC”). It therefore addresses hook load variations directly and is able to reduce weight-on-bit variations during drilling to a minimum. The top mounted DSC/CMC is often supplemented by an active heave compensator cylinder which is used when landing subsea equipment such as BOPs, subsea trees, and during under-reaming and other downhole operations requiring a minimum of motion. The active heave compensator cylinder is mechanically connected to the crown block. Lifting operations are performed by a regular, non-compensated, drawworks. The CMC normally comprises a dual rocker-arm system (for the lifting drawworks) and is capable of handling dynamic loads that are significant compared to the static capacity of the derrick and crown block arrangement. For example, for a derrick, drawworks and CMC each having a static capacity on the order of 1279 tonnes, the CMC dynamic and active capacity is normally on the order of 680 tonnes, i.e. around 50% of the static capacity. The CMC passive cylinder is typically on the order of 7.6 metres.
Another known alternative to the above mentioned DSC/CMC is an active compensated drawworks, i.e. without a top-mounted DSC/CMC. This type of drawworks is typically driven by hydraulics or electrical motors, and the active compensation is performed by a controlled manipulation of the motors and/or hydraulics (pumps, control valves, etc.), based on input data from e.g. a vessel motion recording unit, and causing the drawworks to pay out or reel in wire. This system has no passive mode. An active compensated drawworks is also susceptible to mechanical malfunction, leading to a compete loss of drill string compensation. However, an active compensated drawworks is advantageous compared to the top mounted DSC/CMC in a weight and balance perspective: while the DSC/CMC is comparably heavy and positioned at the top of the derrick, the active compensated drawworks is lighter and arranged at deck level.
The present applicant has devised and embodied the invention in order to overcome shortcomings of the prior art and to obtain further advantages.